"The extensive equatorial forests of the Amazon and Orinoco, and their tributaries, constitute _par excellence_ the home of the American Monkeys, but the majority of the genera have a very extended range, appearing in one or more species throughout the greater portion of the tract covered by the entire family. This is more particularly the case with the Sapajous (_Cebus_), Spider-Monkeys, Howlers, and the species of _Callithrix_. The range of the species, on the other hand, is not unfrequently very sharply defined, as, for example, when a natural barrier, offering insurmountable obstacles to further migration, suddenly interposes itself. Examples of such limitation, as brought about by the dominant water-courses of the equatorial forests," are numerous. Mr. Wallace cites the case of certain species of Saki Monkey (_Pithecia_), found on either side of the Amazon river, whose range, either southward or northward, appears to be limited by that river. "The number of species of these American Apes found in, and north of, the Isthmus of Panama is ten, of which only one (_Ateles vellerosus_) extends into Mexico; _Mycetes villosus_, the Guatemalan Howler, or 'Mono,' has thus far been found only in Guatemala and Honduras. It is a little surprising that the range of only two of the species--the Black-faced Spider-Monkey (_Ateles ater_) {129}and one of the Night-Apes (_Nyctipithecus vociferans_)--extends beyond Colombia, in South America." "None of the South American Monkeys appear to pass west of the Andean chain of mountains south of Ecuador, and even north of the Peruvian boundary the number of such transgressional forms is very limited. Indeed, even among the wooded slopes, a habitation along the basal line of the mountain axis seems to be much preferred. The greatest altitude at which Monkeys were observed by Tschudi in Peru was 3,000 feet (_Lagothrix humboldti_); _Ateles ater_ and _Cebus robustus_ were found at 2,500 feet. On the other hand, Salvin and Godman state that in the district of Vera Paz, in Guatemala, the 'Mono' or Howler is most abundant at an elevation of 6,000 feet; and on the Volcano of Atitlan, in the same country, Mr. Salvin found troops of the Mexican Spider-Monkey (_Ateles vellerosus_) in the forest region of 7,000 feet elevation. "The range of the Marmosets and Oustitis (_Hapalidæ_) is nearly co-extensive with that of the Monkeys proper." (_Heilbrin._) The Pigmy and the Silky Marmoset range as far north as Mexico. THE MARMOSETS AND TAMARINS. FAMILY HAPALIDÆ.